In recent years, with the progress of miniaturization, thinner and lighter designs, and higher facilities, of electronic equipment, various electronic components to be used in these electronic equipment have also been miniaturized and made thinner, thus calling for an active technical development of printed circuit board, onto which these electronic components are to be mounted, to allow high-density mounting of electronic components.
Especially in these days, with the rapid progress of mounting technology, there is an increasingly strong demand for the supply of low-cost multilayer interconnection circuit board on which semiconductor chips such as LSI can be mounted with a high density. In such multilayer interconnection circuit board, it is important that the multilayer interconnection circuit boards be provided with a high reliability of electrical connection between wiring patterns formed with a fine pitch on plural layers respectively and with a superior high frequency characteristics.
With the conventional multilayer interconnection circuit boards in which electrical connection between layers of wiring is made by conventional through-hole structure fabricated by drilling and plating processes, it has become extremely difficult to satisfy requirements of such higher performance and higher facility electronic equipment, and hence circuit board having a new structure or a manufacturing method that allows high-density wiring are being developed to meet these requirements.
A typical example of such development can be found in the multilayer circuit board having all-layer inner via holes as disclosed in Japanese Laid-open Patent Hei 6-268345, wherein inner via holes are filled with a conductor for improved reliability of connection instead of plated copper conductors formed on the inner wall of through holes as has been commonly practiced in connecting wiring layers in the conventional multilayer printed circuit boards, and inner via holes (IVH) are formed immediately underneath component lands or between arbitrary wiring layers thereby realizing miniaturization of the circuit board and high-density mounting.
As a substrate for such a multilayer printed circuit board with an all-layer IVH structure, a substrate for printed circuit board fabricated by impregnating epoxy resin as an insulating material in a fibrous base material such as an aramid non-woven fabric is in general use. These circuit formation substrates have been employed in various electronic equipment that require small size and light weight because of their advantages of a low expansion coefficient, a low dielectric constant, and light weight.
However, as represented by the latest small portable telephones and digital video cameras which have been made remarkably smaller and lighter in weight for domestic use, the available functions have become extremely diversified; and in electronic equipment requiring microminiaturization, it has become necessary to mount a number of LSI chips on a small-area of printed circuit board with a high density. As a result, the chip-on-board technology with which LSI bare chips can be connected to bumps on a circuit board has become indispensable. In implementing this technology while maintaining a high level of reliability, there are extremely severe requirements on the substrate for printed circuit board such as mechanical characteristics including shock resistance and electrical characteristics such as dielectric constant and dielectric loss tangent in a high frequency band. The following problems which have been acceptable in domestic electronic equipment in general are drawing attention as serious problems.
1) Coming off of LSI bare chips due to poor reliability of connection between the LSI bare chips and the circuit board and to poor adhesion between the copper foil for wiring and the substrate, when subjected to a shock such as through dropping of an electronic equipment.
2) Decrease in dimensional stability due to heat experienced during die bonding of LSI bare chips.
3) Decline in dielectric and other characteristics due to humidity absorption of circuit board.
4) Substrate is flammable because of the core material used such as aramid fiber inside the circuit board; when one tries to provide flame retarding property, electrical characteristics and resistance against adverse environment will be lowered.
The above problems may be attributed to the combination of a number of conditions in configuring a substrate for printed circuit board, for example, composing a substrate with an insulating material comprising polymeric materials such as an epoxy resin main component of bisphenol group in combination with a curing agent based on an acid anhydride, or amine or phenol, or to the means of flame retardation based on conventional technology.